The present invention relates to a silicon nitride powder which is readily sinterable to present a sintered body excellent in high temperature flexural strength and toughness, a silicon nitride sintered body obtained therefrom, and processes for their production. The silicon nitride sintered body is a material excellent in the high temperature strength, hardness, corrosion resistance and impact resistance and expected to be widely used as various structural parts. For example it is expected to be useful for gas turbine parts, nozzles or bearings.
2. Discussion of Background
As conventional methods for the production of silicon nitride powders, (1) a method of directly nitriding metallic silicon, (2) a method of reducing and nitriding silica and (3) a silicon halide method are known. The powders prepared by these methods have substantial differences in the sinterability of the powders or in the properties of the sintered bodies after the sintering such as the high temperature flexural strength, probably because of the different production histories, even when the amounts of metallic impurities, the amounts of oxygen, the particle sizes or specific surface areas are similar.
Generally, the powder produced by the method (1) is readily sinterable, but is poor in the high temperature flexural strength, the powder prepared by the method (2) is hardly sinterable, and the powder prepared by the method (3) is said to show an intermediate performance. Such nature is largely dependent on the oxygen content in the form of a solid solution, which constitutes about 60% of the total amount of oxygen.
Since a grinding step is involved in the preparation of powder according to the method (1), the oxygen content in the form of a solid solution usually exceeds 2% by weight, in many cases, and at least 1.5% by weight of such oxygen is contained. In the method (1), the oxygen content in the form of a solid solution can be reduced by conducting a step of e.g. acid treatment for the removal of impurities. Even then, it is difficult to reduce such oxygen to a level of less than 1.0% by weight. On the other hand, in the case of the powder according to the method (2), silica will remain as a silica powder is used as the starting material, and it is usual that the oxygen content in the form of a solid solution exceeds 2% by weight.
With respect to the crystal forms, there are two types i.e. an .alpha.-phase and a .beta.-phase. It i said that the higher the content of the .alpha.-phase, the better from the viewpoint of a general sintering mechanism of silicon nitride, such that the .alpha.-phase is dissolved once in a liquid phase and subsequently becomes super-saturated and precipitates as the .beta.-phase. However, the growth of prismatic crystals during the precipitation of prismatic crystals resulting from the transition of the .alpha.-phase to the .beta.-phase, is not uniform, and there is a drawback that fine densification will be hindered by abnormal crystal growth.
The above-mentioned powders are about all that are presently available. As a matter of course, the amount of oxygen in the powder is influential over the sinterability of the powder and over the properties of the sintered body. Further, various other powder properties such as the specific surface area, the crystallinity, the particle shape, the granularity (fine powder), etc. are involved. Thus, it is not clearly understood how the powder properties according to the above-mentioned respective methods are related to the powder properties.
Japanese Examined Patent Publication No. 43311/1986 discloses the relation between the amount of oxygen in the silicon nitride powder and the high temperature flexural strength. This prior art proposes to reduce the amount of oxygen in the silicon nitride powder to present a sintered body excellent in the high temperature flexural strength. However, (i) no clear description is given as to various properties other than the oxygen content, such as the amount of fine powder which is inevitable in the method of grinding a silicon nitride, the metallic impurities and the specific surface area, probably because a hot press (HP) sintering method is employed, (ii) in the method for producing a powder having a low oxygen content, the special treatment brings about a high cost, and HP which is not commonly used, is employed as the sintering method, as mentioned above, (iii) no detailed discussion is made as to the reason for specifying the species of the sintering aid and as to the structure of the sintered body, which may be attributable to the performance of the high temperature flexural strength, (iv) the high temperature flexural strength is at a level of 600 MPa at best, and (v) there is no disclosure of the influences of the oxygen content in the form of a solid solution and the oxygen content in the form of a hydroxide or oxide, over the high temperature flexural strength.
On the other hand, Japanese Unexamined Patent Publication No. 145380/1990 discloses a relation between the conversion rate to .beta.-phase and the particle size within a range of the .beta.-phase content of from 30 to 100%. However, in this method, the total oxygen content is high, and accordingly the transition from .alpha. to .beta. during the sintering tends to take place easily even at a low temperature. Further, there has been a problem that the amount of the grain boundary phase constituted by the sintering aid is likely to be large, whereby the solubility of silicon nitride changes, and only .beta.-prismatic crystals having a very small aspect ratio will be formed, and the toughness will not be improved.